The invention includes embodiments that relate to an energy storage device. The invention includes embodiments that relate to an energy storage device with a cathode having high packing density and/or high metallic content.
Rechargeable batteries using sodium as the negative electrode are known in the art. Sodium has a standard reduction potential of −2.71 volts. The sodium anode may be used in liquid form, and the melting point of sodium is 98° C. An ion conducting solid electrolyte (separator) separates the liquid sodium anode from a positive electrode (cathode).
A second, molten electrolyte transports ions to and from the separator on the cathode side. The melting point of the molten electrolyte, along with the temperature-dependent, sodium-ion conductivity of the solid electrolyte, determines the minimum operating temperature of the battery. The cathode should include an active metal having a halide species that is compatible with the solid electrolyte in the charged (oxidized) state. Low solubility of the oxidized cathode material in the molten electrolyte can lead to passivation of the remaining uncharged (reduced) electrode surface and fouling of the pores.
It may be desirable to have an energy storage device that has improved operating life, energy density, and power density over those devices that are currently available.